The ability of zinc oxide to absorb ultra-violet radiation is well known. However, zinc oxide produced by standard methods is a white, opaque powder and as such is unacceptable in appearance for use in certain applications where effective absorption of ultra-violet radiation is desirable, e.g. sunscreens, paints, varnishes, plastics, cosmetics etc.
In theory, reducing the particle size of zinc oxide to below the wavelength of visible light will result in visible light being transmitted through the zinc oxide thereby causing it to appear virtually transparent when in a dispersed form e.g. a dispersion in oil. At the same time however, such a small particle size will more effectively scatter and absorb ultra-violet light.
Methods have been developed in an attempt to provide zinc oxide having a reduced particle size so as to render it transparent when in a dispersed form.
The so-called French process comprises oxidation of zinc metal vapour by mixing with air and quenching with excess air. The particle size of the resulting zinc oxide can be decreased by increasing the rate of mixing and quenching. However, the process inevitably produces a proportion of unreacted zinc metal in the zinc oxide product, and the smaller the particle size of the oxide product the higher the level of zinc metal impurity.
Zinc metal is an extremely undesirable impurity, particularly when the zinc oxide contaminated with it is to be used in products such as sunscreens, paints, plastics etc., because it is capable of reaction with air, moisture and organic media to generate undesirable gaseous products. In addition, it tends to impart a grey tinge to the product which is aesthetically undesirable, and is often coarser than the zinc oxide thereby tending to impart a gritty feel to the product.
An article by S. Tichy, SOFW--Journal 119. Jahrgang, August 1993, discloses "micronized" zinc oxide produced by first precipitating the basic carbonate from a purified solution of zinc sulphate or zinc chloride, washing and filtering the carbonate, and finally subjecting it to calcination. The process involves a number of separate steps each of which tends to give rise to a loss in yield of the final product.
The "micronized" zinc oxide is transparent and allegedly has a particle size of about 20 nm and a surface area in the range 50 to 150 m.sup.2 /g. However, scanning electron micrograph studies of the "micronized" zinc oxide have shown it to have a particle size of around 1 .mu.m and to have a sponge-like, or internally porous, structure which accounts for the high surface area.
In addition, when zinc chloride is used in the process, the zinc oxide product tends to be contaminated with chloride which is undesirable, particularly in cosmetics applications, for example, where purity of ingredients is essential.
Another process for making small zinc oxide is disclosed in an article by Liu et al., Journal of Materials Science 21 (1986) 3698-3702. This process comprises dissolving zinc acetate dihydrate in methanol, atomising this solution into very fine (around 2 .mu.m) droplets using an ultrasonic atomiser, and passing these first into a low temperature electric furnace and subsequently into a high temperature electric furnace. One disadvantage with this process is in the use of methanol as a solvent, since this is both expensive, particularly in the high volumes used compared to a relatively small amount of zinc acetate, and dangerous due to its high flammability. Another disadvantage resides in the use of ultrasonic atomisation since this is both difficult and expensive to apply to a large sale.
The zinc oxide produced comprises slightly porous, spherical particles having a mean diameter of 0.15 .mu.m and having a surface area of about 50 m.sup.2 /g. Each particle consists of an agglomerate of a number of smaller individual, or "primary", particles each having a diameter in the range 100 to 200 nm. The relatively high surface area quoted is that of the agglomerate and includes the exposed surface area of each of the "primary" particles.
It would be desirable to produce zinc oxide having a small particle size and that is free of zinc metal impurity by a process that is simple, safe and economical when applied to a large, or production, scale.